MOFs are primarily famous for their tremendous capacity to adsorb gases, but this is actually just one of a number of things their structures lend themselves to be exceptionally good at. Another is housing ‘guests’ of various sizes within its pores. Unlike adsorption, these particles are much larger, and not held in place by electrostatic interactions, but because they physically cannot escape the structure built around them. This is useful if you’d like to protect what’s inside (such as a sensitive therapeutic agent), or a nanoparticle you would rather keep (see Figure 1).
Catalysts are frequently used as nanoparticles because the higher the surface area to volume ratio (and thus the smaller) they are, the greater the catalytic activity. The problem with this is that nanoparticles suspended in a liquid (or indeed a gas) are extremely difficult to get back. Ultrafiltration, centrifugation and electrophoresis are all valid methods in the laboratory, but spectacularly ill-suited to industrial reactors. What this means is that despite catalysts being, by definition, reusable, in reality most or all of a nanoparticle suspension is lost in every cycle. In some cases, such as in the pharmaceutical industry, leaving the catalyst to contaminate the downstream solution simply is not an option, and colossal effort is needed to remove it.
Ideally, one would prefer the catalytic activity of a nanoparticle immobilised onto something that can be easily recovered. There are many techniques to do this, the most common being to attach your nanoparticle to the surface of a polymer resin. While all have their strengths and weaknesses, ultimately these approaches suffer from either gradual leeching of the catalyst, low catalytic activity, or both.
For a long time, MOFs have presented themselves as candidates here: they are modular, tailorable, and are well documented to readily accept a range of nanoparticles. The downside is that MOFs form as micron-scale powders, which aren’t much easier to recover than the nanoscale catalysts they’re trying to protect. Our ability to produce MOFs as larger monoliths changed that.
To demonstrate the concept, we selected a well-known MOF called ZIF-8: it is highly stable, easy to synthesise, and has large cages with narrow windows. We assembled the MOF around nanoparticles of tin oxide (SnO2) – a potent photocatalyst that is good for degrading toxic organic molecules but owing to its size, very difficult to recover and reuse. We then used this composite to break down methylene blue – a toxic dye widely used in the textile industry and frequently dumped into the environment. The experiment was successful: in three hours under simulated sunlight, the composite was able to degrade 94% of the dye. We then tested the recyclability of the catalyst and found no difference in its activity over 10 cycles – a dramatic improvement over current technologies (see Figure 2).
Interestingly, methylene blue is actually much larger than the pore apertures of ZIF-8, meaning that it is not entering the structure to be acted on directly by the catalyst in the way we might expect. Instead, we found that in this case water enters the structure, where it reacts with the photoexcited catalyst and forms hydroxyl and superoxide radicals. These are then free to diffuse back out of the MOF and react with the dye. This technique can therefore in some cases be applied even when the target molecule is large.
In subsequent experiments, we were able to incorporate other nanoparticles into other MOFs, showing the technique is generally applicable. It is also green and surprisingly easy: our SnO2@ZIF-8 composites needed a 10-minute reaction at room temperature and worked on almost the first attempt; 3D graphene aerogels, by contrast, need 12 hours or more at 90 oC and are far less forgiving. We have barely scratched the surface of what we can do with this technology: industrial catalysis is a diverse field and the impacts of this could be significant and wide-ranging. Choosing the right MOF to support a given reaction is one of the first challenges we must address when solving a problem like this.